DYNAMAXThe W grade describes low-temperature performance, while the second number is a hot-viscosity grade, not a temperature or quality ranking. Choose within engine permission rather than automatically thickening for summer, traffic or age.
The two parts of 5W-30
W means winter; 5 is not five degrees Celsius or a viscosity measured at 40°C. The 30 is a hot grade related to viscosity classification at 100°C, not a thirty-degree summer limit. Oils generally thicken when cold and thin when hot; multigrade oil does not become thicker when heated.
Reference: Mobil 1 Racing Oils Product Guide
Multigrade markings are classifications, not complete viscosity values. Two 30-grade oils may have different typical kinematic values, and W is not a printed Celsius range. Cold starting and pumping are related but distinct; a 40°C value is not the W criterion. Read the two directions and match finished-oil requirements; 5W-30 is not five-to-thirty-degree permission.
Reference: Mobil 1 Racing Oils Product Guide
What grade comparisons tell you
| Comparison | What it indicates | What it does not prove |
|---|---|---|
| 0W-30 versus 5W-30 | Different winter grades, both hot grade 30 | Identical performance and OEM codes |
| 5W-30 versus 5W-40 | Same W grade, different hot classifications | 40 is better for every engine |
| 5W-40 versus gear oil 75W-90 | Different classification systems | Direct numeric comparison or interchangeability |
Different W grades do not establish room-temperature feel, nor does a shared W guarantee identical starts. Compare data with matched methods/units, retaining typical-value limits. Engine and gear scales cannot be subtracted for thickness differences. This prevents numeric shopping without home freezing, burning or touch tests.
HTHS differs from kinematic viscosity
HTHS describes high-temperature, high-shear viscosity. Data-sheet kinematic viscosity at 40°C or 100°C is a different measurement with different units. ACEA categories have distinct HTHS requirements; higher is not universally better because design and efficiency requirements also matter.
Reference: ACEA 2023 Oil Sequences for Light-Duty Engines
HTHS commonly uses dynamic-viscosity units, while kinematic values use mm²/s or cSt; direct comparison is meaningless. It is not a maximum temperature or synthetic-content gauge. Do not replace a required low-HTHS category with a higher value for thickness, or lower a required value for economy. Meet performance and engine conditions together.
Choose from the permitted range
Check engine code, mandatory performance and permitted grades before considering the environment. Cooling systems manage temperature; summer does not automatically call for 50 or 60. Investigate cooling and level for temperature warnings. Use OEM conditions when several grades are permitted rather than ranking 0W-50 as universally best.
Reference: API Motor Oil Guide · Mobil: Engine Oil in Hot Weather
Begin with permitted grades; hot weather does not override a single specified fill. Cooling manages temperature, while special load still needs OEM-directed checks and scheduling. Cold travel requires destination and permission checks. Correct oil cannot repair radiators, fans or supply faults, and changing grade is not a diagnostic method.
Reference: Mobil: Engine Oil in Hot Weather · API Motor Oil Guide
A worked candidate comparison
If a car requires 5W-30 and an OEM code, an undocumented synthetic 5W-40 cannot replace it solely because of the larger number. Two 5W-30 oils can differ in HTHS and emissions requirements. Read complete documentation rather than judging oil by feel or pouring speed.
Use requirement/candidate A/candidate B columns for grade, mandatory claims, codes and data. Compare packs, price and supply only after both are eligible. Missing evidence needs checking, not selection by thickness, price or span. Multiple permitted grades may carry conditions; this sheet also helps a workshop explain its recommendation.
Changing grade and handling faults
Quantify consumption and inspect leaks and mechanical condition before thickening oil in an older car. Record old/new products, requirements and quantities, observing level, warnings and starts. Stop safely for pressure warnings rather than experimenting with thicker oil.
Reference: Ford Owner's Manual: Engine Oil Check
Before thickening for consumption or noise, log level/symptoms and investigate causes. Consistent measurements prevent quantity changes from appearing as grade effects. Changed sound does not establish restored pressure or wear. Record product, claims, fill, filter and permission conditions; warnings require diagnosis rather than trying grades until one feels better.
Frequently asked questions
Q1. Is the W number viscosity at 40°C?
No. It identifies a winter grade, not the data-sheet kinematic viscosity at 40°C.
Q2. Is 30 necessarily less heat-resistant than 40?
No. Grade does not establish oxidation or thermal performance; check finished requirements.
Q3. Does 0W mean water-thin oil?
No. 0W describes winter performance; hot viscosity depends on the hot grade and formulation.
Sources and applicable limits
- Mobil 1 Racing Oils Product Guide — Explanation of W and high-temperature grades; racing-product recommendations are not applied to ordinary road vehicles. (Accessed 2026-10-08)
- ACEA 2023 Oil Sequences for Light-Duty Engines — A/B and C categories and aftertreatment requirements; not a universal OEM approval. (Accessed 2026-10-08)
- API Motor Oil Guide — API/ILSAC categories, marks and compatibility; does not verify a particular product license. (Accessed 2026-10-08)
- Mobil: Engine Oil in Hot Weather — Hot-weather oil and cooling-system explanation; specific vehicle grade requirements remain necessary. (Accessed 2026-10-08)
- Ford Owner's Manual: Engine Oil Check — Oil level, filling and supplemental-additive restrictions; a model-specific example, not a rule for every vehicle. (Accessed 2026-10-08)
